Silicone rubber is a synthetic polyorganosiloxane elastomer made from a cross-linked silicone polymer that is generally reinforced with silica, such as fumed silica. Polydimethylsiloxane (abbreviated as PDMS) is a widely used silicone polymer for such purposes. Silicone rubbers are widely used in various demanding applications, such as those requiring high and low temperature stability, a wide range of hardness, chemical resistance, weatherability, electrical properties, compression set resistance, and the like. They are usually prepared by curing suitable curable silicone rubbers. Most of the currently known cured silicone rubber compositions have a low transparency to light, example, less than about 88 percent transparency, when measured on a 2.3 millimeters thick plaque using ASTM D1003 test method. Cured silicone rubbers having a high transparency to light, example, greater than 90 percent with a 2.3 millimeters thick plaque are known, however, the cured sheets made from these materials have inferior mechanical strength, i.e., less than about 4.5 megapascals, as measured using ASTM D612 test method. Some attempts have been made to increase transparency by using silicone rubbers comprising diphenylsiloxane repeat units and matching the refractive index with that of the silica filler. However, the methods are not desirable from a cost and environmental perspective, partly because the preparation of the diphenyl monomer, such as for example, octaphenylcyclotetrasiloxane has environmental issues, due in part to the possibility of generation of polychlorinated biphenyls. Irrespective of the nature of the silicone rubbers, curable silicone compositions used for preparing the cured silicone rubber compositions have high shear viscosities of greater than about 1500 pascal-second (when measured at 10 radians per second in accordance with ASTM D440 test method), which makes them difficult to be pumped, which is a disadvantage, especially from the perspective of a commercial scale process.
Therefore, there is a continuing need for methods for producing easy-to-use curable silicone rubber compositions that have a low shear viscosity, example, less than about 1500 pascal-second, when measured as described above. Further, there is a need for producing cured silicone rubber compositions and finished parts from such curable compositions, wherein the cured compositions have a high transparency of greater than about 90 percent light transmittance, an elongation of at least about 200 percent, and a tensile strength of greater than 4.5 megapascals.